(1) Field of the Invention
The present invention relates to a cobalt base alloy. More particularly, the invention relates to a cobalt base alloy having in combination a high strength and a high ductility at high temperatures.
(2) Description of the Prior Art
Nozzles of gas turbine engines are used at such high temperatures as 800.degree. to 1000.degree. C, over a period of 20,000 to 30,000 hours, and they receive high thermal stress by violent heating or cooling at the time of starting up or shutting down. During the operation, they also receive heat stress caused by non-uniform temperature distribution. As a material for a gas turbine engine nozzle used under these severe conditions, there have heretofore been used cobalt base alloys, and investigations are now being made to develop cobalt base alloys having higher thermal fatigue resistance.
Most attempts heretofore made to develop novel cobalt base alloys are directed to improvements of the strength at high temperatures, especially the creep rupture strength. Namely, it has been considered that development of alloys having a high creep rupture strength will be in conformity with development of alloys having a high resistance to the thermal fatigue. However, nozzles prepared from cobalt base alloys heretofore developed are still insufficient in the resistance to the thermal fatigue and cracks are often formed in these nozzles.
We conducted research on formation of cracks in these cobalt base alloys and found that not only the high temperature strength but also the high temperature ductility is a factor having important influences on the resistance to the thermal fatigue in cobalt base alloys.
As pointed out hereinbefore, gas turbine nozzles are used at operation temperatures of 800.degree. to 1000.degree. C. The time required for elevating the temperature to this level at starting of a turbine is generally about 10 to about 20 minutes. Accordingly, the nozzle material receives a very high thermal stress at the time of starting. Further, even during the operation, the temperature is often changed and the nozzle material also receives a thermal stress during the operation of the turbine. These thermal stresses are gradually accumulated as residual stress. The residual stress increases with the lapse of time but the strength resisting this residual stress, namely the high temperature strength, is gradually reduced with the lapse of time. Accordingly, a crack is formed when the nozzle material cannot resist the residual stress any more. This residual stress causing cracking can be reduced if the material has a high ductility. More specifically, the generated thermal stress is moderated in a short time by the ductility of the material and accumulation of the stress can be remarkably reduced. Therefore, a material of a gas turbine nozzle should have, in combination, a strength resisting the residual stress, namely a sufficient high temperature strength, and a strength moderating the thermal stress, namely a sufficient high temperature ductility.
Conventional cobalt base alloys which have been developed as a result of attempts to improve only the high temperature strength, namely the creep rupture strength, for example, alloy No. P illustrated in the Example given hereinafter, are insufficient in the ductility, especially the ductility at high temperatures exceeding 900.degree. C.